As is known in the art, circuitry such as that described in U.S. Pat. Nos. 6,788,151 and 7,482,869 includes a radio frequency (RF) amplifier which can be operated at high efficiency because direct current (DC) power dissipation can be reduced by tracking a change in an envelope of an RF input signal received by an RF amplifier. However, a relatively complex circuit is needed to provide a reference signal representing a desired power supply voltage that corresponds to the envelope of the RF input signal. In some cases, the reference signal is generated by detecting the input RF signal, rectifying it to generate an envelope of the input signal, and subsequently providing the envelope signal to a supply voltage adjustment circuit while at the same time providing it to a power supply selection circuit to select a desired supply voltage. In this technique, the circuits all have to be synchronized which can be relatively difficult. Furthermore, this technique is performed with analog signals. Consequently, the circuits are sensitive to variations in the operating characteristics of devices resultant from temperature changes and also to differences in device operating characteristics (e.g. due to manufacturing tolerances) which reduces yield of the circuitry.